A magnetoresistive head is used as a read sensor in high recording density magnetic recording technology focusing on hard disk drives and largely influences magnetic recording technological performance. In recent years, it has been well known that the magnetoresistance effect of a multilayered film in which ferromagnetic metal layers are stacked through a nonmagnetic metal layer, the so-called giant magnetoresistance (GMR) effect, is large. In this case, electric resistance changes depending on relative angle of magnetizations of two ferromagnetic layers with a nonmagnetic interlayer. For the use of this giant magnetoresistance effect in magnetoresistive sensors, a structure called a “spin valve” has been proposed. The spin valve includes a structure comprised of an anti-ferromagnetic layer, a ferromagnetic layer, a nonmagnetic interlayer and a ferromagnetic layer where a exchange coupling field generated in the interface between the anti-ferromagnetic layer and ferromagnetic layer substantially pins magnetization of the ferromagnetic layer in contact with the anti-ferromagnetic layer and magnetization of the other ferromagnetic layer is freely turned by an external magnetic field to obtain output. Hereinafter the ferromagnetic layer the above magnetization of which is substantially pinned by the anti-ferromagnetic layer is called first pinned layer and the ferromagnetic layer the above magnetization of which is turned by an external magnetic field is called free layer. The abovementioned basic structure is common to various types of GMR currently in practical use, more specifically CIP-GMR, tunneling magnetoresistive sensors (TMR) and current-perpendicular-to-the-plane GMR (CPP-GMR).
The aforementioned magnetoresistive sensors are structures developed to improve SNR (signal/noise ratio) of the magnetic head. For improvement of SNR of the magnetic head, it is necessary to increase the signal and reduce the noise. Magnetic noise such as Barkhausen noise and electric noise such as Johnson noise and shot noise are known as magnetic head noise. Barkhausen noise occurs because magnetization of free layer has a magnetic domain. Barkhausen noise can be controlled by providing bias layers at both sides of the multilayered film of the magnetic head in the track width direction. On the other hand, electric noise mainly depends on resistance. Therefore, in order to improve SNR of the magnetic head, efforts have been pursued toward magnetic domain control to suppress generation of magnetic domains, etc, and toward higher power output, typically by decrease of magnetic film resistance and development of new magnetic film such as TMR.